The US Espionage Act is a little-used law dating from 1911 – the same year as the UK's Official Secrets Act, with which it has much in common.

The current law can be used to prosecute those who leak and publish classified information that creates a national security risk. But experts say the purpose of the act is primarily to tackle espionage, and that it has never been invoked successfully against a media organisation.

Although insiders who have leaked sensitive information have been prosecuted under the law – including two former officials of the American-Israel public affairs committee accused of leaking information in 2007 – cases have rarely resulted in convictions.

Media organisations are unlikely to be prosecuted under the act, under the constitutional protections for free speech upheld by the supreme court in a number of cases that have limited the application of the Espionage Act.

Experts say that in addition to protections for free speech, there are difficulties with proving leaked documents are classified, under a US government executive order which sets limits on documents that can be properly termed as classified documents.

However, if charges are made against Julian Assange under the law in the US, then he would face extradition under the controversial UK-US extradition treaty, which provides special measures for extraditions between the two countries.